Optical waveguide directional couplers are well-known and much-relied-on sub-components for integrated opto-electronic devices. Being an analogue of the bulk-optics semi-transparent mirror, they provide a means for apportioning light between (or remixing) two paths and any split-ratio (the most common being 50:50) can be obtained.
The standard directional coupler consists of two optical waveguides which run parallel, and sufficiently close together for their guided fields to interact, for a distance L before diverging to stop the interaction. The coupling coefficient (k) is a falling exponential function of the waveguide spacing and varies strongly with the waveguide confinement factor (which is an expression of the degree to which the radiation energy is confined with the waveguide channel)--itself a sensitive function of almost every waveguide parameter.
This sensitivity is a major problem with the use of such couplers. It makes device performance quite unpredictable and uncontrollable in practice unless the couplers are tuned interactively at some stage in manufacture.
An example of such tuning concerns GaAs/AlGaAs waveguide devices as shown in FIG. 6. Such waveguide is of the rib type having a rib 2 formed in a layer 4 of AlGaAs. Layer 4 is disposed on a layer 6 of GaAs, forming a guide layer, which in turn is disposed on a substrate 8 of AlGaAs. The confinement factor--and hence coupling coefficient--is sensitive to the residual thickness .delta. of the AlGaAs layer 4 into which the rib 2 has been etched: i.e. a composite function of rib height and original layer thickness, both of which quantities are subject to random variations in standard processing (a nominal accuracy of 10% is usually asserted). Since facets must be formed for optical input/output, the test cannot be done in-situ; thus tuning involves cleaving a test-piece out of the wafer and launching light through a test directional coupler. Wet chemical etchant is then dropped onto the test piece and the variation of output levels monitored until the desired condition reached when it is quickly rinsed. Simply timing this etching and then treating the rest of the wafer similarly, frequently gives uneven results due to non-uniformity of the wafer. For best results each individual device must be tuned separately--clearly unacceptable in production. InP waveguides for opto-electronic integration have proved more difficult to wet-etch; thus such tuning is more difficult, while the integrated nature of the device demands a high yield of good, well-tuned couplers made by `dead-reckoning`.
A second problem is that directional couplers are sensitive to the optical polarization. TM (E normal to the surface) gives a larger confinement factor than TE (E parallel to the surface) which can amount to a .times.2 different in the coupling coefficient. This is unacceptable if the light is remote sourced via standard optical fibre which inevitably scrambles the polarization.
A third problem is the physical length (L) of the parallel-waveguide section which frequently exceeds 1 mm if the guide spacing is sufficient to ensure that it survives photolithographic reproduction (&gt;2 .mu.m), this aggravates problems in the variation of thickness of the residual thickness of layer 4 of FIG. 6.
The coupling phenomenon is most readily and accurately viewed as an interference between the two modes of a composite guiding system.
Referring to FIG. 1, a composite guiding system comprises as shown in FIG. 1a two spaced waveguides comprising ribs 10, 12 of width W, formed in a layer 14 of InP and spaced by a gap g. The major part of the radiation is guided in an underlying layer 16 of InGaAsP. The symmetric and anti-symmetric modes (FIG. 1a) have amplitude maxima which are in-phase in one waveguide (adding) but in anti-phase in the other (cancelling). Some distance (Lo=.pi./2; known as the coupler transfer length) down-guide this phase relationship will have reversed since the modes travel with slightly different velocities. This results in cancellation in the first guide 10 but reinforcement in the second 12; all the light has coupled over and continues to couple back and forth in a spatially sinusoidal manner.
As the gap between the inner edges of the guides is progressively reduced to zero, the modal velocities become increasingly different and the transfer becomes more rapid spatially. At zero gap the modes are just the fundamental and first-order modes of a double-width waveguide 20 (FIG. 1b); however they interfere in the same way and light input to one side of such a waveguide will excite both modes and will swing from side to side of the waveguide with a spatial period readily predictable from the waveguide parameters.
Such zero-gap or TMI (Two Mode Interference) of BOA (Bifurcation Optique Active) couplers are well known but are usually made relatively long (many beat-lengths) in order to emphasize the wavelength sensitivity of the coupling to obtain a wavelength diplex function.